Different apparatuses are currently known, which are capable of determining the moisture content of a material; these apparatuses may be classified schematically in two groups, according to whether the measurement is made on a continuous or discontinuous basis.
Among the apparatuses falling within the first group, it is possible to mention, for example, neutron probes and retrodiffusion probes. These permit results to be obtained rapidly, but their lack of accuracy at low moisture content levels and their sensitivity to the surrounding medium limit their use considerably.
In the second group, mention will principally be made of the apparatuses making use of weight measurement methods, in which an operation of heating a sample is followed by a weighing operation. (U.S. Pat. No. 4,168,623, U.S. Pat. No. 3,909,598, JP-A No. 58-162839 for ex.) These apparatuses give satisfactory results, but the drying time is long and becomes further extended with increasing initial moisture content of the material under investigation. Furthermore, for a given moisture content range and a given weight of material, there is an optimum drying power, beyond which the period of time required for drying does not diminish any further.
Finally, these apparatuses are not suitable for carrying out rapid measurements.